Liquid crystals have been adapted already as the electric optical elements for many kinds of devices such as display devices, shutter alley for a printer, shutters for cameras, and are actuated in the display of watches and table calculators.
Most of the liquid crystal display elements presently actualized are elements utilizing the dielectric orientation efficiency of a nematic liquid crystal or a cholesteric liquid crystal. But, concerning the expected requirements for display elements with many picture elements, there are such problems as, an insufficient response ability, a contrast not able to obtain the necessary drive margin, and an insufficient visual angle characteristic. Therefore, extensive research and development of the TFT panel or MOS panel that forms the switching element in each picture element is being carried out.
In this state, Clark et al (U.S. Pat. No. 4,367,924) conceived new a liquid crystal element with new display principle, utilizing the smectic phase, which cures the above defects of the liquid crystal element. The explanation of this new element is as follows;
FIG. 1 is a typical diagram of the smectic C* phase or the smectic H phase. The liquid crystal is composed of molecule layers 1, and in each of the layers, the average direction of the molecule axis is inclined o degrees in the vertical direction to the layers. Meyer et al teaches in a thesis titled "Ferroelectric liquid crystal" of "Physical Journal" (Le Journal de Physique Vol. 36, March, 1975 PPL-69 to L-71), that, the Smectic C* or H phase composed of an optically active molecule generally has an electric dipole density P and is ferroelectric. This dipole density P is vertical to the molecule direction n, and is parallel to the layer surface of the smectic. What they teach is adaptable to the smectic H phase, but in the H phase, the viscosity to the rotation around the axis vertical to the layer, becomes large. The existence of electric dipoles in these chiral smectic liquid crystals gives a stronger coupling force to the electric field, than that of dielectric anisotropy. Furthermore, it can be said that this coupling force has a polarity characteristic, in the sense that the favorable direction of P is the parallel direction to electric field E, and so, by reversing the direction of the applied electric field, the direction of P can also be reversed.
Therefore, by reversing the electric field, as shown in FIG. 2, the direction of the molecule can be controlled by moving the molecule along the cone (the angle 2.degree. of this cone is hereinafter referred to as the cone angle). Then, by detecting the variation of the average molecule axis direction with two polarizers, the liquid crystal element can be utilized an electric optical element.
Because this electric optical element which utilizes the response of the Smectic C or H phase to an electric field, holds a coupling force between its spontaneous polarization and the electric field, a force which is 3 or 4 orders larger than the coupling force of dielectric anisotropy, compared to TN type liquid crystal element, this electric optical element has an efficient high speed response characteristic, and it can also have a memorizing characteristic by selecting the appropriate orientation control; thus it is expected to be adapted as a high speed optical shutter or as a display with a large capacity for information.
Meanwhile, many kinds of chiral smectic liquid crystal material having a ferroelectric characteristic have been studied and synthesized. The first synthesized ferroelectric liquid crystal is called DOBAMBC.
p-Decyloxybenzilidene-p'-amino-2- methylbutyl cinnamate, and many compounds of this series of liquid crystal compositions, shown in the following formula, have been synthesized for research concerning ferroelectric liquid crystals. ##STR3## wherein X represents H, Cl, or CN and Y represents Cl.
But as this series of liquid crystal shows the chiral smectic phase at a temperature higher than room temperature, there are such defects as, not being able to be used at room temperature, it is a Schiff base series, it is decomposed with water, and its stability is bad.
As a developed compound of this series, a Schiff base series chiral smectic liquid crystal compound in which the hydroxyl group is introduced into one of the benzene rings, and provides a hydrogen bond in the molecule, as shown in the following general formula, was announced by B. I. Ostrovskii ("ferroelectric", 24,1980,309) and by A. Hallsby (Molecular crystals and liquid crystals, Letter 82,1982,61), and this developed compound has been given attention as being a compound showing the smectic C* phase at a wide range of temperature including room temperature. ##STR4##
As this compound also has a hydrogen bond within the molecule, it is hardly decomposed with water and has much better stability a compared to general Shiff series of liquid crystal. But as a practical matter, it is required that the compound not crystallize at a temperature below 0.degree. C., and so the liquid crystal material synthesized with compounds of this series are still not completely satisfactory.
Other than this, a liquid crystal material of the azoxy series was announced by P. Keller et al ([Annales de physique]1978, 139) but with this compound too, there are such practical problems as not having efficient characteristics concerning the temperature range, and being a deep yellow color.
Among these compounds, an ester series liquid crystal having a good stability and widely practiced as a TN type liquid crystal material has been given much attention. In a well known document, a liquid crystal compound has been disclosed having a structure of the formula; ##STR5##
This formula was announced by B. I. Ostrovskii et al, as being material showing the chiral smectic liquid crystal at a temperature range close to room temperature. Also, G. W. Gray et al announced in "Molecular Crystal and Liquid Crystal" 37 (1976) 189, (1978) 37, a biphenyl ester series of compounds showing chiral smectic liquid crystal at a high temperature range.
As stated above, at present, a liquid crystal that shows a chiral smectic phase over a wide range of temperature including the practical condition of room temperature, does not exist, and, even with materials showing a chiral smectic phase at a relatively wide range of temperature, there still have been some problems in its stability.
Therefore, the present invention has an object of offering a new liquid crystal which has good stability, and is highly likely to provide a chiral smectic liquid crystal compound at a wide range of room temperature including room temperature.